Secondary ion emission under keV carbon cluster bombardment
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Secondary ion mass spectrometry (SIMS) is a surface analysis technique capable of providing isotopic and molecular information. SIMS uses keV projectiles to impinge upon a sample resulting in secondary ion emission from nanometric dimensions. It is well documented that secondary ion emission is enhanced using cluster projectiles compared to atomic projectiles. Previous studies of enhanced secondary ion yields with cluster projectiles have led to the present study dealing with the scope of C60 as a projectile for SIMS. The secondary ion yields (i.e., the number of secondary ions detected per projectile impact) from impacts of 10-26 keV C24H12+, C60+, gramicidin S+ and C60F40+ projectiles were examined to compare the effectiveness of the projectiles. The [M-H]- secondary ion yields from several organic samples varied inversely with the molecular weight. Multiple ion emission decreases monotonically as a function of the number of secondary ions emitted per impact and varies with impact energy such that higher energies produce more multiple ion emission. The emission of CN- from biological samples as a function of carbon-based projectile characteristics was examined to explore the possibility of using CN- as a molecular identifier. CN- emission was found to be the product of both direct and recombination/rearrangement emission. Re-emitted projectile atoms in the form F- were found under C60F40+ bombardment. Two forms of re-emitted F- were found: One form in which F atoms retained a portion of the initial kinetic energy, and a second in which the F atoms deposited most of the initial kinetic energy into the surface before being ejected. The [M-H]- secondary ion yield of gramicidin S was increased ~ 15 times by embedding the analyte in a matrix of sinapic acid. These results show the optimum carbon based projectile for a given sample is dependent upon the signal to be monitored from the surface. The results also show CN- has potential as a molecular identifier. Additionally, the detection of re-emitted F- confirms prior predictions of re-emitted projectile atoms.
Locklear, Jay Edward (2006). Secondary ion emission under keV carbon cluster bombardment. Doctoral dissertation, Texas A&M University. Texas A&M University. Available electronically from